What is baudline?
Mystery Signal
X marks the spot at 32000

Audio cards at the 32000 sample rate were analyzed for a common X crisscross pattern.  A linear sine wave sweep is the signal displayed in the spectrogram images below.  Signal generation and collection were in full duplex loopback mode with both the ADC and DAC sample rates set to 32000.

This gallery is part of the Full Duplex DAQ comparison survey. 

Cirrus Logic CS4236B Sound Blaster Vibra16X

Sound Blaster 16 PCI Sound Blaster 128 Sound Blaster Live!

VIA 8235 SiS 7012 ESS Maestro 2E

Griffin iMic v0.06 Griffin iMic v3.00

Six of the ten audio devices exhibit the distinct symmetrical X mark and one device has X-like crosshatching.  Why is this artifact so common?

The spot is dead center located at half the Nyquist frequency (8000 Hz) and halfway into the time duration of the linear sweep.  There is nothing special about the position of the spot other than it is exactly in the middle.  This geometry can have a number of DSP explanations.

The positive slope line of the X looks like a frequency inverted (folded) version of the main spectrogram linear sweep fundamental.  Most of these audio devices have good quality ADC and DAC anti-alias filters yet for some reason the fundamental is being inverted and mixed in.  This artifact is probably related to the 48000 / 32000 = 1.5 decimation ratio. 

It is interesting that this phenomena is present in so many audio devices made by different manufacturers.

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